It is widely known to effect surface treatments such as formation of oxide coatings and electrolytic etching on metals such as aluminum by subjecting such metals to electrolysis. These surface treatments are carried out by methods which can be broadly classified as batchwise or continuous. Treatments of the latter type are capable of mass production and, therefore, are adopted for electrolytic treatment of various metallic articles such as building materials and electrolytic capacitors.
In the continuous electrolytic treatment of a thin metallic sheet, for example, it has been customary to feed power directly to the metallic sheet being treated by means of metallic contact roll. Recently a method for the electrolytic treatment of metallic objects by a liquid power feeding instead of the power feeding by direct contact has been proposed.
The term "liquid power feeding" as used herein means a method for indirectly feeding power through the medium of the electrolyte. Since this method does not require direct electric contact at any point on the object under treatment, it is sometimes alternatively called a "non-contact electrifying method". This method is described in detail, for example, in "Working Surface Technique", Vol. 29, No. 10, pp. 17-21 (1982). Electrolytic treatment by the liquid power feeding method is particularly suitable for continuous high-speed treatment of a thin sheet or foil of metal. The feasibility of this particular method of electrolysis in the commercial production of electrolytic capacitors of Al and Ta, for example, is presently underway. In the process of electrolytic treatment using this liquid power feeding method, the treatment for desired formation is obtained by feeding power to an anode disposed in an anodic power feeding compartment and to a cathode disposed in a forming compartment and continuously passing a metallic foil under treatment in an electrolyte which fills both compartments. The metallic foil is polarized between the two compartments and is subjected to electrolytic oxidation while functioning as a cathode within the anodic power feeding compartment and as an anode within the forming compartment. Electrolytic treatment by the liquid power feeding method, therefore, necessitates additional use of an insoluble anode capable of withstanding use in the electrolyte.
As the electrolyte, a solution of an ammonium salt of an inorganic acid such as boric acid or phosphoric acid has been used. Recently, a solution containing an ammonium salt of an organic acid has been demonstrated to yield more desirable results (as described in the "Handbook on Metal Surface Techniques", page 677 (1976) published by Japan Industry News and in Japanese patent application Laid Open No. SHO 56(1981)-140618). So far, no anode has been reported which provides stable service over a long period in an electrolyte containing an organic acid or a salt thereof. Thus, it has been difficult to carry out an electrolytic treatment of a metallic object using the liquid power feeding method with this electrolyte.